1. Field of the Invention
The present invention generally relates to monitoring and testing equipment useful in semiconductor manufacturing and, more particularly, to a portable laser light scattering assembly used for in situ detection of particles formed and suspended during plasma processing within a reaction chamber.
2. Description of the Prior Art
Recent studies in our laboratory have shown that many plasmas produce particles which may be a significant cause of product contamination and device failure. These experiments have shown that particles can be nucleated and suspended in a process plasma until they grow to significant size. Observed particle sizes have ranged from submicrons to hundreds of microns in diameter. Micron scale particle contamination is a serious problem in semiconductor device manufacture. Many device yield loss and reliability problems are attributed to particle contamination from plasma processes. Particle contamination may causes device failure, poor film quality, changes in material resistivity, and impurity permeation. The problems due to particle contamination are expected to increase as device dimensions are reduced in future technologies.
The particles eventually fall onto semiconductor wafers being processed in the plasma environment. If particles fall before or during film deposition or pattern transfer, they could disrupt those process steps. If they fall at the end of a process step, the particles may disrupt subsequent process steps.
The effects of particle contamination can be magnified by selective plasma etching processes, which rely on a combination of feed gases and etching conditions to selectively etch material surfaces on the wafer. The chemical formation of particles which are themselves slowly etched in these highly selective plasmas form a so-called micromask resulting in an irregular surface often referred to as "grass". These spikes and hillocks of unetched material degrade device performance and reduce process yield.
Our experiments have shown that the plasma itself can result in product contamination. It is therefore important to develop means to operate the plasma while controlling or eliminating particle formation. Laser light scattering studies in our laboratory have indicated that the plasma composition and gas flow have a pronounced effect on the formation of particle contamination in etching plasmas. In particular, faster gas flow, resulting in shorter residence time in the plasma, as well as lower gas pressures and shorter plasma exposure all tend to inhibit particle formation in certain plasmas. Feed gas chemistry also has an important effect on particle formation.
While it is now recognized that the largest share of total surface contamination is contributed by plasma tools and processes, few methods exist for controlling process induced particle contamination, and no qualitative and quantative techniques exist for detecting and evaluating this contamination inside a tool vacuum chamber during processing.
Current contamination measurement methods rely on the use of blanket (i.e., unpatterned) monitor wafers processed with product wafers. The monitor wafers are removed from the plasma processing tool and analyzed in a surface contamination monitoring tool. One problem with this post-process analysis is that many wafers have already been processed when the monitor wafers are analyzed, so that this method does not detect contamination problems before they affect lots of product wafers. Also, post-process analysis provides little information as to various sources of contamination, the distribution of particles in the process chamber volume and the process conditions that influence the formation of growth particles.
Clearly, it is highly desirable to monitor particles in situ during processing. In this way, contamination sources can be identified and controlled with the capability for real-time feedback of process contamination data necessary for optimization of process/tool cleanliness.
Laser light scattering (LLS) is an effective means of detecting particles larger than 0.2 .mu.m. This method has been demonstrated for in situ detection of particles in plasma processing tools as reported by G. S. Selwyn, J. S. McKillop, K. L. Haller, and I. J. Wu, J. Vac. Sci. Tech., A8, 1726 (1990), G. S. Selwyn, J. Singh, and R. S. Bennett, J. Vac. Sci. Tech., A7, 2758 (1989), and G. S. Selwyn, K. L. Haller, and J. E. Heidenreich, Appl. Phys. Lett., 57, 1876 (1990). These studies have shown that tool and process design has a marked effect on the level of particle contamination and that the particles are electrostatically suspended at the plasma/sheath boundary. Electrostatic suspension of the particles also simplifies their detection because the particles will be concentrated, more or less, into a plane at the plasma/sheath boundary.